Conventionally, target object detecting apparatuses, such as radar apparatuses, have been performing detections of target objects by transmitting high frequency signals (RF signals) to detection ranges and receiving reflection signals that are the transmission signals respectively reflected on the target objects.
JP4081035B discloses a target object detecting apparatus that forms an RF signal into a pulse-wave shape and transmits the RF pulse signal with a predetermined pulse height only within a predetermined transmission period. Moreover, the target object detecting apparatus sets a period of time in which such an RF pulse signal with the predetermined pulse height is not transmitted as a reception period, and receives a reflection signal within the reception period. Further, recently, A-class and AB-class amplifications are used for purposes of applying to pulse compression processing and a C-class amplification is not used.
In order to generate such an RF pulse signal, an output from a power FET for amplifying an RF signal may be controlled, in which either one of a method of controlling a gate voltage of the power FET and a method of controlling a drain voltage of the power FET which is disclosed in JP4081035B is used.
FIG. 7A shows a circuit of a conventional drain switching circuit 21P for controlling a drain voltage of a general power FET, and FIG. 7B is an operation explanatory view of the general power FET. As illustrated in FIG. 7A, the conventional drain switching circuit 21P includes n-type field effect transistors (FETs) 211P and 213P and a p-type FET 212P. In the FET 211P, a gate is connected with a control pulse input terminal 214C, a source is grounded, and a drain is connected with a drive voltage input terminal 214D via a resistor 217P. The drain of the FET 211P is connected with gates of the FETs 212P and 213P.
In the FET 212P, a source is connected with the drive voltage input terminal 214D and a drain is connected with a drain of the FET 213P. A source of the FET 213P is grounded.
A control pulse signal is applied to the control pulse input terminal 214C, and a drive voltage Vds is applied to the drive voltage input terminal 214D.
A connection point in the drain switching circuit 21p with such a configuration between the drain of the FET 212P and the drain of the FET 213P is connected with a drain of the power FET. Further, an output Dcon of the drain switching circuit 21P is substantially 0 [V] of a LOW state within a period of time in which the control pulse signal is LOW, and a drain voltage of the power FET becomes substantially 0 [V]. The output Dcon of the drain switching circuit 21P is substantially Vds [V] of a HI state within a period of time in which the control pulse signal is H.
However, with the method of controlling the gate voltage of the power FET, although an operational amplifier is generally used, a fall time length of the general operational amplifier is long. Further, with an operational amplifier with high unity gain frequency (ft), although the fall time length is shorter, there is a problem of output waveform ringing, and stability in operation being LOW.
Moreover, the drain switching circuit 21P in FIG. 7A generally uses a MOSFET for each FET. However, the FET 212P used as a high-side FET is a p-type FET of which an input capacity is large and a responding speed of falling is slow. Therefore, as illustrated in the “212P response” of FIG. 7B, it does not shift to an OFF state rapidly at an end timing of a transmission period specified by the control pulse signal. Thus, the drive voltage is supplied to the drain of the power FET even after the transmission period ends (after a reception period starts). Therefore, a part of the transmission signal flows into a receiver, and a receiving sensitivity within a close distance range degrades immediately after the transmission period is switched to the reception period.
The present invention is made in view of the above situation, and provides an RF signal generation switching circuit that can rapidly fall a waveform of an RF pulse signal while operating stably, and an RF pulse signal generation circuit using the switching circuit.